SQL Function to calculate end time based on duration not working

Can you help me with this question about scripting scalar functions in T-SQL? I have some data from a call center that contains a start time and a duration for each call. I want to create a function that will output the end time of the call. Here is a sample of the data:

I've created a function that I think should work, but it always returns the start time without adding anything to it. I want it to parse [DURATION] (varchar), calculate the duration in seconds, then add that to the start time. Here's what it looks like: